Conventional Multi-carrier Code Division Multiple Access Research Articles

English

The main aim of this paper is to investigate the performance of multi-carrier code division multiple access (MC-CDMA) technique, which has gain much attention due to its high frequency spectrum efficiency and high data rate transmission and makes the wireless communication more reliable and efficient. MC-CDMA technique is the combination of both orthogonal frequency division multiplexing (OFDM) and code division multiple access (CDMA), which found to be very effective to mitigate the effect of fading channels and to improve the system performance. In this paper, we investigate the performance of conventional MC-CDMA system, orthogonal wavelet packet based MC-CDMA system (WP-MC-CDMA), and Huang Hilbert Transformation (HHT) based MC-CDMA system. However, the conventional MC-CDMA has already been discussed in the literature, and used as a benchmark for other two schemes. In addition, the orthogonal wavelet packet based MC-CDMA technique is designed with a set of wavelet packets and used as the modulation waveforms in a multicarrier CDMA system. The WP-MC-CDMA shows their superiority over conventional MC-CDMA in terms of bit error rate (BER), and helps to mitigate the effects of interference and channel fading. Moreover, we also investigate the performance of Huang Hilbert Transformation based MC-CDMA. This scheme outperforms than that of other two techniques. This is due to Huang Hilbert Transformation based MC-CDMA is based on the knowledge of the instantaneous channel state information and instantaneous imperfect channel estimates. Therefore, by the knowledge of their instantaneous channel gains, it shows their superiority over of conventional MC-CDMA and WP-MC-CDMA in terms of spectral efficiency and data rate transmission. Furthermore, we also present the comparison all three techniques in terms of BER. Finally, our numerical and simulation results validate our proposed theoretical findings.

English

The multicarrier code division multiple access (MCCDMA) is a strong candidate for the future wireless mobile communication systems put forth by service quality and system capacity. In order to exploit the maximum possible channel diversity and desirable correlation properties, the modification of MCCDMA system is addressed here to pave the way to remove the multiple access interference (MAI) effect and to increase the spectral efficiency (SE) due to the possible integration of space-time block coding (STBC) and multi input multi output (MIMO) techniques with two-dimensional orthogonal complete complementary (OCC) codes. The impact on modifications in the MCCDMA system is studied. Due to frequency selective fading and multipath effects, MCCDMA suffers from MAI due to loss of orthogonality among the users. In this paper, the constellation movement scheme is proposed to break the interference limitation in order to increase the SE and system capacity. In this scheme, the constellation of one of the users is moved adaptively relative to the other by an optimal angle based on quantized estimation to counteract MAI, caused by multipath effects without varying the transmit power in order to satisfy minimum distance threshold at the detectors. The simulation result shows that this modified MCCDMA system and proposed constellation movement scheme achieves significant performance improvement in terms of bit error rate (BER) and SE compared to conventional MCCDMA systems using unitary spreading codes. It also shows that the proposed scheme combats MAI against frequency selective fading and multipath effects and achieves a significant improvement in user capacity as the number of users increased. Key words: Multicarrier code division multiple access (MCCDMA), multiple access interference (MAI), orthogonal complete complementary (OCC) codes, channel states, adaptive constellations.

Performance Improvement of Multi-UserMC-CDMA system Using Discrete Hartly Transform Mapper

Future wireless communication systems must be able to accommodate a large number of users and simultaneously to provide the high data rates at the required quality of service. In this paper a method is proposed to perform the N-Discrete Hartley Transform (N-DHT) mapper, which are equivalent to 4-Quadrature Amplitude Modulation (QAM), 16-QAM, 64-QAM, 256-QAM, … etc. in spectral efficiency. The N-DHT mapper is chosen in the Multi Carrier Code Division Multiple Access (MC-CDMA) structure to serve as a data mapper instead of the conventional data mapping techniques like QPSK and QAM schemes. The proposed system is simulated using MATLAB and compared with conventional MC-CDMA for Additive White Gaussian Noise, flat, and multi-path selective fading channels. Simulation results are provided to demonstrate that the proposed system improves the BER performance and reduce the constellation energy as compared with the conventional system

Design and performance of WH-spread CI/MC-CDMA with iterative interference cancellation receiver

This paper addresses a novel Walsh–Hadamard (WH)-spread multicarrier code division multiple access (MC-CDMA) system which employs carrier interferometry (CI) codes in a multiuser environment. In frequency selective channels, phase characteristics of CI codes ensure better estimates of received WH-spread CI/MC-CDMA signals. Estimation of multiple access interference (MAI) becomes more reliable with time and frequency diversities of CI/MC-CDMA signals with spreading gain diversity of WH codes over multipath channels. Interference cancellation (IC) is done by taking hard and soft estimates of received data bits. Simulation results demonstrate that the proposed multiple access scheme with iterative decoding offer a significant performance gain over WH-spread MC-CDMA and CI/MC-CDMA over multipath channels. We observe that WH-spread CI/MC-CDMA maintains a stable envelope of the transmitted signal as that of CI/MC-CDMA. In an overloaded situation, the proposed multiple access scheme provides a low peak to the average power ratio (PAPR) compared to conventional MC-CDMA for multirate systems which supports simultaneous transmission of high and low data rate users.

CDMA Transmission with Complex OFDM/OQAM

We propose an alternative to the well-known multicarrier code-division multiple access (MC-CDMA) technique for downlink transmission by replacing the conventional cyclic-prefix orthogonal frequency division multiplexing (OFDM) modulation by an advanced filterbank-based multicarrier system (OFDM/OQAM). Indeed, on one hand, MC-CDMA has already proved its ability to fight against frequency-selective channels thanks to the use of the OFDM modulation and its high flexibility in multiple access thanks to the CDMA component. On the other hand, OFDM/OQAM modulation confers a theoretically optimal spectral efficiency as it operates without guard interval. However, its orthogonality is limited to the real field. In this paper, we propose an orthogonally multiplex quadrature amplitude modulation (OQAM-) CDMA combination that permits a perfect reconstruction of the complex symbols transmitted over a distortion-free channel. The validity and efficiency of our theoretical scheme are illustrated by means of a comparison, using realistic channel models, with conventional MC-CDMA and also with an OQAM-CDMA combination conveying real symbols.

Performance Analysis of Multiband Complex Wavelet Based MC-CDMA System with Space Diversity Combining in Rayleigh Fading Channel

In this paper, the multiband complex wavelet used as multi-carrier basis function is optimized. On the basis of analyzing the principle of multi-carrier code division multiple access (MC-CDMA), a new multiband complex wavelet based MC-CDMA (MBCW-MC-CDMA) system model is proposed, and space diversity combining (SDC) technique is employed to improve the system performance further. The uplink performance of proposed MBCW-MC-CDMA system with SDC technique is investigated in Rayleigh fading channel, and corresponding bit error rate (BER) analysis is given in detail. Without any cyclic prefix (CP), the system can avoid the decrease of spectrum efficiency and data rate of conventional MC-CDMA with CP. By comparison, our scheme outperforms real wavelet packet based MC-CDMA and slightly outperforms conventional MC-CDMA with CP via utilizing superior properties of the optimized multiband complex wavelet. Theoretical analysis and simulation result show that the application of SDC technique can improve the proposed system ability to combat spatial fading and various interferences effectively. Moreover, the proposed system based on SDC technique has superior BER performance over conventional DFT and SDC based MC-CDMA system and real wavelet packet and SDC based MC-CDMA system.

Performance of Fractionally Spread Multicarrier CDMA in AWGN as Well as Slow and Fast Nakagami-<tex>$m$</tex>Fading Channels

In multicarrier code-division multiple-access (MC-CDMA), the total system bandwidth is divided into a number of subbands, where each subband may use direct-sequence (DS) spreading and each subband signal is transmitted using a subcarrier frequency. In this paper, we divide the symbol duration into a number of fractional subsymbol durations also referred to here as fractions, in a manner analogous to subbands in MC-CDMA systems. In the proposed MC-CDMA scheme, the data streams are spread at both the symbol-fraction level and at the chip level by the transmitter, and hence the proposed scheme is referred to as the fractionally spread MC-CDMA arrangement, or FS MC-CDMA. Furthermore, the FS MC-CDMA signal is additionally spread in the frequency (F)-domain using a spreading code with the aid of a number of subcarriers. In comparison to conventional MC-CDMA schemes, which are suitable for communications over frequency-selective fading channels, our study demonstrates that the proposed FS MC-CDMA is capable of efficiently exploiting both the frequency-selective and the time-selective characteristics of wireless channels.

Multiple-Access Interference Suppression for MC-CDMA by Frequency-Domain Oversampling

A technique for interference suppression in multicarrier code-division multiple-access (MC-CDMA) systems is proposed which exploits the structural differences in signals that arrive at the receiver with Doppler shifts or carrier offsets. Oversampling the received signal in the frequency domain and properly combining the samples provides the interference suppression. Frequency-domain oversampling is accomplished by using a time extension of the conventional MC-CDMA signal or unconsumed portions of the cyclic prefix. Furthermore, a receiver structure is introduced that despreads and combines groups of samples so that a linear minimum mean-squared error solution for combining the groups is easily found. This combining scheme increases the signal-to-interference ratio experienced by the desired user in the MC-CDMA system. In addition, the receiver performs well in severe near-far scenarios when there is sufficient frequency separation between the signals of the desired user and an interferer. Numerical results show that the proposed receiver significantly outperforms the conventional MC-CDMA receiver in many channel realizations.

Receiver Design for Wavelet-Based Multicarrier CDMA Communications

Wavelet packets have good properties such as orthogonality and multirate flexibility, and have resulted in a number of works for its applications to code-division multiple-access (CDMA) communications. In this paper we investigate the use of a special set of the wavelet packets, i.e., the waveforms formed from a full binary wavelet packet tree, in multicarrier CDMA systems. In the conventional multicarrier CDMA systems, a simple frequency-domain combining receiver is commonly used to achieve frequency diversity. To combat multipath channel effects, guard intervals are inserted between consecutive symbols. On the other hand, wavelet packets have the property of localization in both time and frequency domains. A novel receiver design utilizing this property to achieve both frequency and time diversity is proposed and evaluated in this paper. This receiver eliminates the need for guard intervals. Simulations are given to support the system and receiver design. This analysis can be extended to the more general wavelet packet basis waveform sets.

Performance of an MC-CDMA system based on the complex wavelet packet and pre-equalisation technique

A novel complex wavelet packet-based MC-CDMA (CWP-MC-CDMA) system is proposed, based on the analysis of the principle of multi-carrier code division multiple access (MC-CDMA) and the use of the optimised complex wavelet packet. By making use of the reciprocity between uplink and downlink channels under the time-division duplex (TDD) schemes, the authors adopt the pre-equalisation technique to simplify the receiver architecture and suppress the multi-access interference (MAI) effectively. The system performance is investigated in multipath Rayleigh fading channels. The CWP-MC-CDMA scheme with the energy-controlled pre-equalisation technique is also developed due to the greater energy consumption in pre-compensating for the weak sub-carriers. The theoretical analysis and simulation results show that the proposed CWP-MC-CDMA system has a superior bit-error rate (BER) performance to that of the conventional MC-CDMA system based on DFT (DFT-MC-CDMA), and the system with pre-equalisation also outperforms other detection-combining techniques, such as equal gain combining (EGC), maximum ratio combining (MRC), orthogonality restoring combining (ORC) and minimum mean square error combining (MMSEC).

Interleaving scheme for multicarrier CDMA system

In MultiCarrier Code-Division Multiple-Access (MC-CDMA) system, the received signals scattered in the frequency domain are combined to get frequency diversity gain. However, the frequency diversity gain is limited because of correlation between subcarriers. A novel interleaving scheme for MC-CDMA system is proposed in this paper. A circular shifting register is introduced into each subcarrier branch to decrease the correlation between subcarriers. By using interleaving, frequency diversity gain of system is increased. System structure and model with interleaver are discussed. In the case of multiple users, Parallel Interference Cancellation (PIC) technique is also introduced. Computer simulations demonstrate the performance of proposed scheme, and the performance comparison of MC-CDMA with interleaver and conventional MC-CDMA system is shown as well.

Performance of multicarrier CDMA using divided sequence frequency diversity over multipath fading channel

A new multicarrier code division multiple access (MC-CDMA) system is proposed. In this system, both frequency and time diversity can be obtained by using divided spreading sequences and a Rake receiver. Simulation results show that the proposed MC-CDMA system has a high level of robustness against multipath effects and outperforms conventional MC-CDMA systems. The influence of changing the number of S/P branches and copied data on the average bit error rate is investigated.

Performance of MC-CDMA based on wavelet packets in Rayleigh multipath fading channel

A novel multi-carrier code division multiple access (MC-CDMA) scheme based on wavelet packets, called WP-MC-CDMA, is proposed. The performance of the WP-MC-CDMA scheme is investigated for a channel with Rayleigh multipath fading. The numerical and simulation results show that the performance of the WP-MC-CDMA scheme is significantly better than that of the conventional MC-CDMA (CNV-MC-CDMA) scheme.

A multicarrier CDMA system with adaptive subchannel allocation for forward links

Multicarrier transmission schemes have been introduced into code-division multiple access (CDMA) systems to gain advantages for high data rate transmission. One of the methods is to transmit identical narrowband direct-sequence (DS) waveforms in parallel over a number of subchannels using frequency diversity. In this paper, we propose a multicarrier CDMA system with an adaptive subchannel allocation method for forward links. In the proposed system, instead of identical DS waveforms being transmitted over a number of subchannels in parallel, each user's DS waveform is transmitted over the user's favourite subchannel which has the largest fading amplitude among all the subchannels. We analyze the performance characteristics of the system when orthogonal and random signature sequences are used. The proposed system is shown to have performance gain over the conventional multicarrier CDMA system. We also investigate how the performance is influenced when the signal is not perfectly allocated into the best subchannel.